1. Field of the Invention
The present invention relates to improvements in systems which involve the sparging or bubbling of a gas into a hot liquid for any one of a variety of purposes, such as deodorizing aeration, liquid oxidation reaction (LOR), hydrogenation, or other action, in which the effectiveness or efficiency of the system is dependent upon mass transfer through a gas-liquid interface which, in turn, is dependent upon the surface-to-volume ratio of the gas bubbles. Smaller gas bubbles have a larger surface-to-volume ratio and are less buoyant than larger bubbles and therefore provide a greater gas-liquid interface and dwell time for producing the desired results such as dissolution, oxidation-displacement, chemical reaction or other gas-liquid interchange.
2. Description of the Prior Art
Gas sparging or bubbling through hot liquids, such as edible oils and other melted oleaginous materials, is commercially employed for a variety of purposes, and reference is made to commonly--owned U.S. Pat. Nos. 4,919,894; 5,004,571; 5,009,816 and Re: 32,562. These representative patents disclose various Advanced Gas Reactor (AGR) gasification and recirculation systems which employ a draft tube as an impeller-surround to draw a gas down from an overhead gas space into an impeller to mix it with the bulk liquid for the intended purpose.
The gas bubbles formed in such AGR systems, by single or multiple impeller agitation of the liquid and/or by subsurface introduction of fresh gas as shown in U.S. Pat. No. 5,004,571, do not have a large surface-to-volume ratio. A single passage of the gas through the liquid does not provide a satisfactory gas-liquid interchange, and therefore the AGR systems depend upon continuous recirculation of the gas from the overhead gas space, and agitation through the impeller, to produce the desired gas-liquid interchange. Suction of the overhead gas down into the impeller is dependent upon the level of the liquid within the vessel, so that system can be troublesome well as inefficient.
It is also known other commercial aeration-type systems to utilize pipe spargers, sintered metal spargers or injectors with various nozzles.
Mass transfer through the gas-liquid interface is quite often the controlling factor in gas-liquid reaction and stripping operations. Smaller bubbles have a larger surface-to-volume ratio than large bubbles, and therefore, reaction or mass transfer will proceed faster with smaller bubbles than with larger bubbles. Therefore, various types of spargers are used to introduce fine bubbles into a liquid. However, the temperature of a hot liquid can be substantially higher than the temperature of the injection gas. For example, the temperature of an edible oil under deodorization conditions can be as high as 650.degree. F. The gas being injected at room temperature will form bubbles as a function of the orifice size and pressure. As a small bubble rises through the hot oil, it is heated up rapidly to the operation temperature, and the volume of the gas expands with the rise in temperature. The expanded bubble has a very small surface to volume ratio, resulting in an undesirable reduction in mass transfer rate.
The problem associated with expanding bubble size is significant, particularly if gas consumption is critical. For example, the nitrogen consumption has to be kept to a minimum in order for a nitrogen deodorizer to operate economically. Motive is required in vacuum jets to create high volume for operating a nitrogen deodorizer. If the flow rate of the non-condensable nitrogen increases, the motive steam requirement will increase substantially. In that case, the nitrogen deodorizer may no longer be competitive with the steam deodorizer.
In hydogenation or oxygenation reactions, gas bubbles rise from the bottom of the tank to the liquid surface and are lost unless a recycle mechanism such as used in the LOR or AGR systems reuses the headspace oxygen or hydrogen. However, the reaction rate can be improved if the gas is dissolved in the first pass. Smaller bubbles, without thermal expansion, will dissolve at a faster rate due to high interfacial area. With increased oxygen or hydrogen dissolution the selectively and amount of byproduct formation may also change. For a large process, a 10% improvement in selectively and rate can be translated into increased efficiency and economy.
Deodorizers, such as for edible oils as disclosed in U.S. Pat. No. 5,241,092, generally operate under vacuum and at high temperatures. Mechanical agitation is not feasible under such conditions since the integrity of the seals would be threatened.